Methods are described for the synthesis of a series of pyrimidine nucleosides and nucleotides that are constrained in known conformations (anti:gauche-trans and anti:trans-gauche) but which are otherwise close structural analogs of naturally-occurring and chemotherapeutically-active nucleosides. These compounds will be used as conformational probes in a study of the relationship between enzyme-affinity and nucleoside conformation aimed at the development of antimetabolites having greater affinity and specificity for their target enzymes than those currently available. The conformationally-restricted nucleosides will be examined in vitro for their ability to act as substrates and/or inhibitors of certain enzymes that are important in the therapy of cancer with antimetabolites. This research will include enzymes involved in activation and degradation of chemotherapeutically active nucleosides (kinases, phosphorylases, deaminases) as well as those thought to be the primary sites of inhibition (thymidylate synthetase, polymerases). The restricted nucleosides will be evaluated in a variety of anticancer screens. The conversion of the restricted nucleosides into their 5'-phosphate esters in intact L1210 leukemia cells will be examined. The behavior of these nucleosides in whole-cell systems may give some insight into the relationship between nucleoside conformation and the biological factors such as uptake and transport that are important in determining selectivity of chemotherapeutic action in vivo.